1. Field of the Invention
The present invention relates to an image forming apparatus for fixing an unfixed toner image by heating, and a temperature control device for a fixing unit for use therewith.
2. Related Background Art
Conventionally, in the image forming apparatuses using a ceramic heater having branches as heat generating means for fixing, the branches for branched heater were switched in accordance with the used paper size. The switching is performed irrespective of whether the heater is conducting or not.
However, there is a drawback that if the switching of heater branch end is made during conduction, the spark or noise will occur at the switch contact point for switching, adversely affecting an electric circuit.
Also, an image forming apparatus has been heretofore devised in which using pressure transport means (roller) which is driven for rotation while pressing an unfixed toner image on the transfer medium against a heater consisting of a heat generating resistor having a plurality of branches, the toner is fixed onto the transfer medium by placing the transfer medium into close contact with the heater via a film moving at the same speed as a transport rate of the transfer medium. In such apparatus, in a mode in which multiple image forming operations such as a multicopy are consecutively effected by one operation, the fixing heater is controlled at the same temperature until a set number of image forming operations are ended.
Accordingly, there is produced a variation in the temperature distribution of the fixing heater because of the difference between the time for which the transfer medium passes therethrough and the time for which it does not pass, causing a problem that the film may shift due to that variation.
A conventional image forming apparatus for thermally fixing the toner with a ceramic heater having a pattern of heat generating resistor having a plurality of branches involves controlling the temperature with one temperature detecting element placed at a predetermined position. Further, the branching conduction is switched only with the size of used copying paper.
Accordingly, owing to variations in the heat generating resistor pattern (variations in the thickness of pattern or resistor material), the temperature distribution of heat generating resistor is not uniform, in some cases giving rise to a fixing failure or having detrimental effect on the film slippage control.
Also, the conventional temperature control for a so-called roller fixing unit makes the control of applying a maximum electric power until a predetermined temperature is reached, turning off the conduction to heater upon detecting the predetermined temperature or greater, and supplying again the maximum electric power below the predetermined temperature.
In the case of roller fixing, such control was sufficiently practical because of a great heat capacity of the heat roller, but there is a problem that when the temperature of fixing unit is low, the fixing can not be effected immediately after conducting, and in the light of such characteristic, the usage of fixing unit to make the temperature control of fixing unit at any time, irrespective of whether the fixing operation is performed, has been widely made, presenting several problems broadly from the aspects of economy and ecology.
On the other hand, among the fixing units as above described, there is developed a fixing unit comprising a member moving along with a thin film belt (film) of great heat conductivity and a heater of small heat capacity. Such fixing unit can effect the temperature control in the range from a sufficiently low temperature to a fixing temperature in a short fast copy time. Herein, when the temperature control of heater is made, it is necessary to suppress the ripple (overshoot) in raising the temperature of heater up to a predetermined temperature. Thus, conventionally, the application power (voltage) is controlled depending on the difference between the temperature detected by a temperature detection element attached on the heater portion and a predetermined temperature.
However, there is a problem that only by making such control that the application power (voltage) is changed depending on the temperature detected by the temperature detection element attached on the heater portion, the overshoot may be reduced, but because this control does not take into consideration other factors different than the heater temperature, the first transfer sheet after starting conduction may have poor fixing ability, principally due to the nature that the heat capacity of heater is small.
On the contrary, to improve the fixing ability, if the electric power applied to the heater is increased, there is the disadvantage that the overshoot may be larger, but the heater of small heat capacity mainly made of ceramic is damaged, resulting in a shorter life and less durability of an apparatus.
Further, it has been found that because the temperature control is not well performed, the transfer sheet undergoing fixing may curl up, albeit excellent fixing ability and durability, causing a jam.
Also, the factors involved in the fixing ability and durability include the heater temperature, the pressure roller temperature, the responsibility of temperature sensor for the heater, the size of copying paper, the thickness of copying paper, the quality of material, the outside air temperature, the humidity, and the service condition of the main body in the past, but if these factors are adapted for the control to solve the above problems relating to the fixing ability and the durability, there is the drawback that the number of parameters will increase, and the relation between all the parameters and the amount of control is difficult to formulate.
That is, where the number of parameters (status amounts) is increased to determine the control amount for the heater electric power in attempting the optimal power control for the heater, or where there exists any parameter which has ambiguous relation with the control amount among the parameters, the relation between the parameters and the control amount is difficult to formulate.